LIBRARY OF CONGRESS. 

_ 

Sb.elf,_Cai.9 



UNITED STATES OF AMERICA. 



THE 



>MEDIC7m:pHDEP'^ 



ESSENTIALS OF PHYSICS, 



1 B : 

CONDICT W. CUTLER, M.D.. 

LATE HOUSE PHYSICIAN, BELLEVUE HOSPITAL &c. 




F.ntered according to act of Congress, in the year 1884. 

BY CONDICT W. CUTLER, 

In the Office of the Librarian of Congress, at Washington. 



" Banner " Steam Print, 
Morristown, N. J. 



CONTENTS. 

BOOK I.— ON MATTER. 



Chapter. 
I. 


Matter. 


II. 


Liquids. 


III. 


Gases. 




BOOK II.— ON HEAT. 


Chapter. 
I. 


Thermometers, &c. 


II. 


Expansion. 


III. 


Fusion and Latent Heat.. 


IV. 


Vapors. 


V. 


Hygrometry. 


VI. 


Transmission of Heat. 


VII. 


Calorimety. 


VIII. 


Sources of Heat, and Cold. 


IX. 


Mechanical Equivalent of 
Heat. 



BOOK III.— ON LIGHT. 
Chapter. 

L Propagation of Light &c, 
II. Intensity of Light. 

III. Reflection of Light. 

IV. Refraction of Light. 



4 
V. Transmission of Light. 
VI. Dispersion and Achromation 
of Light. 
VII. Optical Instruments. 
VIII. Photography. 
IX. Double Refraction. 
X. Sources of Light. 



BOOK IV.— ON MAGNETISM AND 

ELECTRICITY. 
Chapter. 

I. Magnetism. 
II. Theories of Electricity, &c. 

III. Statical Electricity. 

IV. Electrical Machines. 

V. Effects of Electrical Dis- 
charge. 
VI. Dynamical Electricity. 
VII. Chemical Batteries. 
VIII. Direction and Measurement 
of Electrical Currents. 
IX. Effects of Current Electri- 
city. 
X. Electro Dynamics. 
XI. Voltaic Induction. 
XII. Thermo-Electric Currents. 



APPENDIX.— ON METEOROLOGY. 



BOOK I. 

CHAPTER I. 

Physics treats of the phenomena present- 
ed to us by matter. 

Matter is anything we can appreciate by 
our senses, and is made up of sixty-seven 
elements or substances that cannot be 
divided into other substances. 

PROPERTIES OF MATTER. 

a. Impenetrability. The property of occu- 
pying space exclusively. 

b. Extension or volume. The space matter 
occupies. 

c. Form. The definite shape of matter. 

d. Indestructibility. The inability to de- 
stroy matter even though it may change its 
form. 

e. Elasticity. The tendency for mat- 
ter to return to its original form or volume 
when the force that altered its shape ceases 
to act. 



DIVISION OF MATTER. 

Matter may be divided into:— 

Atoms. 

Molecules. 

Masses. 

a. An Atom is the smallest division of 
matter and unite to form molecules. 

b. A molecule is the smallest group of 
atoms that will exhibit the physical and 
chemical properties of the substance and 
cannot be divided without losing these 
characteristics. 

c. A mass is a collection of molecules. 
Molecules present themselves in three 

different states, as — 
Solids. 
Liquids. 
Gases. 

a. In a Solid state the molecules are 
naturally in a fixed condition and as a con- 
sequence solid bodies tend to retain a given 
form. 

b. In the Liquid state the molecules 
move freely upon each other. 

c. In the Gaseous state the molecules 
tend to repel each other or to occupy 
greater space. 



FORCES GOVERNING MATTER. 

1. Force controlling atoms. 

a. Chemical affinity or that force which 
causes atoms to combine or to remain com- 
bined. 

2. Forces controlling molecules, 
Cohesion, 

Adhesion, 
Repulsion, 
Polarity. 

a. Cohesion is that force which unites 
molecules to form masses. 

b. Adhesion is the molecular force which 
binds together the surfaces of two or more 
masses. 

c. Repulsion is that force which causes 
molecules to occupy more space. 

d. Polarity is that force which unites 
molecules to form crystals. 

3. Force controlling masses. 
Gravitation is the force attracting masses 

to each other and is governed by the fol- 
lowing law. 

The attraction is directly in proportion to 
the mass and inversely as the square of the dis- 
tance. 



Gravity is the force exerted by the earth 
upon a body. 

SPECIFIC GRAVITY. 

Specific Gravity or Density is comparative 
weight. 

The standards for comparison are— for 
liquids and solids, water at a temperature ol 
6o° F. and for gases and vapors, air or hy- 
drogen. 

The principle by which we obtain the 
specific gravities of substances, depends 
upon the law of Archimedes. A body im- 
mersed in a liquid loses weight equal to the 
weight of the liquid displaced. 



CHAPTER II. 

LIQUIDS. 

Liquids are but slightly compressible and 
according to Pascal's law — Pressure is trans- 
mitted undiminished in all directions, and 
at right angles to all equal surfaces. 

1. Means of obtaining specific gravity of 
solids. 

By Hydrostatic balance. Weigh the sub- 
stance first in air, then in water, and divide 
its weight in air by its loss of weight in 
water. The result is its specific gravity. 

2. Of Liquids. 

a. By the specific gravity bottle. 
Subtract the weight of the bottle from the 

weight of the bottle filled with water and 
then from the weight of the bottle filled 
with the liquid, and divide the latter by the 
former, the result is its specific gravity. 

b. By the hydrometer, which consists of a 
graduated glass tube loaded at the bottom 
with mercury so it may sink to a certain 
depth in fluids, the depth depending upon 
the specific gravity of the substance. 



CHAPTER III. 

GASES. 

In gases the molecules tend to expand or 
repel each other. Otto Degarager first es- 
tablished the fact that gases had weight. 

The pressure or weight of the atmosphere 
at the level of the sea is about fifteen 
pounds to the square inch, but diminishes 
proportionally as we ascend. 

Torricelli 's experiment. 

Torricelli first determined the pressure of 
the atmosphere by taking a glass tube seal- 
ed at one end, filling it with mercury and in- 
verting it by placing the open end covered 
with the thumb in a vessel of mercury. 

He found by this experiment that the 
mercury in the tube remained about thirty 
inches above the level of the mercury in 
the vessel and the weight of this mercury 
represented the pressure of the atmosphere 
to be in proportion of fifteen pounds to the 
square inch. 

The Torricellian vacuum is the space above 
the mercury in the tube. 



Pascal determined that the height of the 
mercury in the tube really did depend upon 
the pressure of the atmosphere, for perform- 
ing the same experiment on a high moun- 
tain, he feund the mercury did not remain 
so high. 

BAROMETERS. 

Barometers are instruments used for mea- 
suring atmospheric pressure. 

Many of them are based on the principle 
of Torricelli's experiment — the pressure of 
the atmosphere being measured by the 
height of a column of mercury. 

BAROMETRIC CORRECTIONS. 

Corrections have to be made in mercury 
barometers for : — 
Temperature. 
Elevation, 
Capillarity. 
State of weather. 

a. Temperature causes either contraction 
or expansion of the mercurial column 
itself. 

b. Elevation above the le /el of the sea 
causes the column to fall. 

c. Capillarity causes the mercury to ad- 
here to the sides of the tube. 



d. In dry weather the atmosphere is 
dense and consequently the pressure on 
the mercury causes it to stand higher than 
it does in wet weather when the air is rarer 
and contains more aqueous vapor. 

AIR BAROMETERS. 

Beside fluid barometers there are air ba- 
rometers, Aneoid's being a good example. 

This consists of a thin metallic box ex- 
hausted of air, the top of which is so thin 
that it yields readily to alterations in the 
pressure of the atmosphere. These altera- 
tions are transmitted by levers to an index 
which indicates the pressure on a scale. 

LAW OF GASES. 

a. Boyles Law. The volume of a gas is 
inversely as the pressure. 

b. The rabidity with which gases mix is 
inversely as the square root of their density. 

c. The density of a gas is directly as the 
pressure. 

SPECIFIC GRAVITY OF GASES. 

Weigh the same volume of air and the gas 
and divide the weight of the gas by that of 
the air. The result is the specific gravity 
of the gas. 



AIR PUMPS. 

The principle of the air pump depends 
upon the expansive or repulsive property 
of gases. 

We take away a portion of air and the re- 
mainder expands so as to fill up the whole 
space. 

In this way the air becomes greatly rari- 
fied, but an absolute vacuum cannot be 
produced. 

In Spren^el's air pump which depends 
upon the principle of converting the space 
to be exhausted into a Torricellian vacuum, 
falling mercury passes the aperture of the 
vessel to be exhausted of the gas. 

The gas as it were is entangled in the 
falling mercury and in this way a nearly 
complete vacuum is produced. 

It is on the principle of Sprengel's air 
pump, that traps in water pipes are syphon- 
ed, thus leaving the pipes free to allow the 
passage of sewer gas. 



BOOK II. 

CHAPTER I. 

HEAT. 

Theories. 

1. Emission Theory. Heat is considered 
an imponderable fluid which surrounds all 
molecules and the entrance of this fluid into 
substances causes heat and its egress cold. 

2. Undulatory Theory. Heat is due to a 
rapid vibratory movement of the molecules 
of any substance. That all space is filled 
with an imponderable ether which hot 
bodies set in rapid vibration, thus commu- 
nicating a more rapid vibration to the mole- 
cules of adjacent substances. 

This;theory is the one generally adopted. 

TEMPERATURE. 

Heat causes all substances to expand, 

hence by the different expansive qualities 

of liquids, solids and gases, we can measure 
temperature. 



i5 
Temperature is the greater or less extent 
a substance tends to impart sensible heat to 
other substances. 

THERMOMETERS. 

Temperature is measured by thermome- 
ters. Liquid thermometers are generally 
used — mercury and alcohol being the best 
suited for this purpose. Mercury, because 
it boils only at a very high temperature, 
and alcohol because it does not freeze even 
at the lowest temperatures. 

MERCURY THERMOMETERS. 

In constructing an ordinary mercury 
thermometer, two fixed points are deter- 
mined for graduation — viz : 

a. Boiling point of water at the level of 
the sea. 

b. Freezing point of water. 

As these two temperatures are constant 
and the expansion of mercury is nearly uni- 
form, the graduation of the thermometer is 
accomplished by dividing the space between 
the freezing and boiling points of water into 
an equal number of parts called degrees. 

The size of these degrees differ in various 
countries. 



i6 

In this country the Fahrenheit scale is 
used — freezing point 32 — boiling point 

212°. 

In France and on the Continent general- 
ly, the Centigrade scale — freezing point 
taken as o° and boiling point ioo°. 

In some portions of Germany the Rean- 
mure scale is chiefly used— the freezing point 
being o° while the boiling point is taken 
at 8o°. 

CONVERSION OF DEGREES. 

i. Fahrenheit to Centigrade. 
(F-32 ) J = C. 

2. Fahrenheit to Reaumur. 
(F. - 32 ) J = R. 

3. Centigrade to Reaumur. 
\ C=R. 

4. Centigrade to Fahrenheit. 
£ C + 32 - F. 

5. Reaumur to Fahrenheit. 
\ R + 32 = F. 

6. Reaumur toCentigrade. 
I R. = C. 



17 

ALCOHOL THERMOMETERS. 

Alcohol thermometers are used for deter- 
mining low temperatures only, for alcohol 
boils at 78 C. and as it nears the boiling 
point it expands very irregularly thus des- 
troying its accuracy for high temperatures. 
As mercury freezes at 40 C. and alcohol 
does not freeze at the lowest temperatures, 
it is here that alcohol thermometers are 
found so useful. 

They are graduated by means of compari- 
son with the mercury thermometer at differ- 
ent degrees of temperature. 

AIR THERMOMETERS. 

Air thermometers are based on the ex- 
pansion of air by increase of temperature. 
This expansion is denoted by the rise and 
fall of a colored liquid in the tube of the 
thermometer. The graduation of the scale 
is made by comparing the position of the 
fluid with the indications of a mercury 
thermometer at equal temperatures. 

Thermometers may be 

Delicate. 

Accurate. 

a. A delicate thermometer is one that 



i8 

measures rapidly very small changes ot tem- 
perature. 

b. A thermometer is accurate when it 
indicates the changes of temperature cor- 
rectly. 

PYROMETERS. 

Pyrometers are instruments used for 
measuring very high temperatures. They 
are various in kind. 

a. Some are made of metals that melt at. 
a known temperature. 

b. Others depend upon the principle of 
the air thermometer. 

c. Still others on the electric thermom- 
eter — higher the temperature the better the 
conductility. 



CHAPTER II. 

EXPANSION. 

1. Of Solids. 

Solids expand in three dimensions. 

a. Linear. 

b. Superficial. 

c. Cubical. 

The co-efficient of expansion is the expan- 
sion due to the rise of temperature from 
o° to i° C. The co-efficient of expansion in 
solids vary to a marked degree, and from 
this fact many applications are made. 

Thus in the compensation pendulum the dis- 
tance between the center of oscillation and 
the center of suspension remains constant 
at all temperatures by properly using metals 
with different degrees of expansibility, in 
its construction. 

2. Of liquids. 

The co-efficient of expansion of a liquid is 
the increase of the unit of volume for a 
single degree. 

With an increase of temperature there is 



an increase of volume. In water however 
if the temperature fall below 4 C. it ex- 
pands till the freezing point is reached. 

Hence the maximum density of water is 
at 4 C. 

3. Of gases. 

The co-efficient of expansion of a gas is its 
increase of volume for i° C. 

This co-efficient is nearly uniform for all 
gases at all degrees of temperature. 

Gases expand ^ °f their volumes for 
each degree Centigrade. 



CHAPTER III. 

FUSION. 

Fusion is the passage of a body from the 
solid into the liquid state. 

Every substance begins to fuse at a cer- 
tain temperature and that temperature re- 
mains the same until fusion is complete. 

All substances that expand on passing 
from the solid to the liquid state, as the 
great majority do, any increase in pressure 
will raise the melting point. The reverse 
is true in all substances — such as ice, that 
contract during fusion. 

LATENT HEAT. 

Latent heat is that heat which causes 
change of state or fusion and *s not indi- 
cated by the thermometer. Thus if one 
pound of water at 8o° C. be mixed with one 
pound of ice at o° C. the result will be 
water at o° C. Hence it takes as much heat 
to melt one pound of ice as will raise one 
pound of water 8o Q C. Therefore 8o° C. is 
called the latent heat of water. 



During: solution a quantity of heat 
becomes latent. It is on this principle of 
fusion that artificial cold is produced. 

In passing- from a solid to a liquid state 
a certain amount of heat is required and 
the extraction of this heat from surround- 
ing substances lowers their temperatures. 

Chemical affinity accelerates fusion, hence 
acid and a salt or ice and salt acting in this 
way are better freezing mixtures than a salt 
or ice alone would be. 



CHAPTER IV. 

VAPORS. 

Vapors are aeriform fluids into which vo- 
latile substances are changed by the absorp- 
tion of heat. 

Vapors are transparent and usually color- 
less. Vaporization is the rapid passage of a 
liquid into the gaseous state. 

Evaporatio?i is the slow production of va- 
por from the free surface of a liquid. 

Under ordinary conditions liquids pass 
but slowly into the gaseous state, but in a 
vacuum all volatile liquids are immediately 
converted into vapors. 

Heat also hastens vaporization. 

Saturated vapors. When a vapor is so 
dense that any increase of vaporization or 
pressure, or any decrease of temperature, 
will cause a portion of it to assume a liquid 
state the vapor is said to be saturated. 

The higher the temperature the more va- 
por it takes to reach the point of saturation. 



24 

Tension. The pressure exerted by a gas 
is called its tension, The tension of a gas 
is measured by the pressure it exerts on a 
column of mercury. Increase the tempera- 
ture and the tension of a gas is increased 
proportionally. 

Boiling is the rapid production of bubbles 
of vapor in the mass of liquid itself. Boil- 
ing takes place when the tension of its va- 
por is equal to the pressure of the atmos- 
phere, hence any increase in the pressure 
will raise the boiling point, and any dimi- 
nution in the pressure will lower the boil- 
ing point. 

The former fact is made use of in Papins 
digestor and in the patent soup kettles. 

The boiling point is the temperature at 
which a liquid boils. 

EVAPORATION. 

Evaporation is aided by 

a. Heat. 

b. Dryness of surrounding atmosphere. 

c. Increase of surface. 

d. Renewal of atmosphere. 

LATENT HEAT OF VAPORS. 

As it requires a certain amount of heat 



25 

not indicated by the thermometer to turn a 
solid into a liquid, so it does to convert a 
liquid into a vapor. 

i The latent heat of evaporation of water is 
540 C, and this heat has to come from sur- 
rounding substances, such as heat produced 
hy rapid combustion, &c. Hence evapori- 
zation produces cold, and by appropriate 
apparatus water, mercury, &c, can be fro- 
zen by it. 

LIQUEFACTION OF VAPORS AND GASES. 

Liquefaction of vapors and gases is ac- 
complished by means of 

a. Cold. 

b. Pressure. 

c. Chemical affinity. 

Distillation is an example of liquefaction 
of vapors by means of cold. All gases such 
as Hydrogen, Nitrogen and "Laughing 
Gas" can be liquefied by pressure. 

SPHEROIDAL CONDITION. 

When a drop of water is thrown upon a 
red hot surface, as iron, it does not boil or 
rapidly evaporate, but forms a globule that 
rotates rapidly over the iron. This is called 
spheroidal state and is due to the globules 



26 

resting upon a cushion of its own vapor 
produced by the heat radiating from the hot 
surface to its under surface. Many liquids 
have a spheroidal state. 



CHAPTER V. 

HYGROMETRY. 

Hygrometry is determining the quantity 
of aqueous vapor contained in a given 
quantity of air. 

The air is never saturated, but the ratio 
of the quantity of aqueous vapor to the at- 
mosphere when saturated is called the hygro- 
metric state. 

There are a number of instruments made 
: or determining this state called hygrome- 
ters. 

These are, 

Chemical hygrometers. 

Condensing " 

Absorbing " 

Psychrometers. 

a. Chemical hygrometers are based on the 
property that some substances have of ab- 
sorbing all the water in the air passed 
through them. 

The substance, such as the chloride of 
calcium having been weighed before and al- 



28 

ter the passage of the air through it, the 
difference in weight represents the amount 
of aqueous vapor present. 

b. Condensing hygrometers are based upon 
the fact that if the temperature of the at- 
mosphere is lowered, at a certain degree the 
point of saturation will be reached. 

This temperature at which the air is satu- 
rated is called the dew-point. 

By comparing the temperature of the 
surrounding atmosphere and the dew-point, 
and dividing the tension of the atmosphere 
at dew-point by the tension of the surround- 
ing air at the given temperature, will give 
the hygrometnc state of the air. 

c. Absorption hygrometers are based upon 
the property which some organic sub- 
stances have of elongating when moist, and 
shortening when dry. 

d. The Psychrometer or wet-bulb hygro- 
meter is based upon the fact that a fluid 
evaporates more rnpidly in the air, in pro- 
portion as the air is drier, and consequently 
the temperature of the fluid falls in the 
same proportion. 

The instrument consists of two thermo- 



2 9 

meters, the bulb of one covered with wet 
muslin. 

The nearer the air is to being saturated 
the nearer the two thermometers will indi- 
cate — the wet-bulb thermometer always in- 
dicating the lower temperature. 



CHAPTER VI. 

TRANSMISSION OF HEAT. 

Heat is transmitted by 
Radiation. 
Conduction. 
Convection. 

a. Radiated heat is that which is trans- 
mitted to a body from the source of heat 
without affecting the temperature of the in- 
tervening medium and its intensity is in- 
versely as the square of the distance and di- 
rectly as the temperature of its source. 

Bodies that transmit radiated heat readily 
are called Diathermanic, while those that 
stop radiated heat, Athermanic. Bodies that 
absorb the most heat are the best radiators, 
the two properties being equal. 

b. Conducted heat is that which is trans- 
mitted in the mass of the body itself. Bodies 
conduct heat with very different degrees of 
facility. Metals are the best conductors 
while organic substances and liquids are 



3i 

Door conductors. Gases conduct heat very 
ittle if at all. 

c. Convected heat is that which is trans- 
mitted in liquids. When liquids are heated 
at the bottom, ascending and descending 
currents are produced by the heated layers 
becoming less dense and rising while the 
denser and colder layers above sink to the 
bottom. This method of transmitting heat 
is called convection. 

REFLECTION OF HEAT. 

The laws governing the reflection of heat 
are like those of light, {See chapter on light.) 

Some bodies have much greater reflecting 
power than others. Metals have the great- 
est and lamp-black the least. 

White bodies reflect heat well but absorb 
little ; the reverse is true of black sub- 
stances. 



CHAPTER VII. 

CALORIMETRY. 

Calorimetry is the measure of the quantity 
of heat a body loses or gains during change 
of state or temperature. 

Quantities of heat in substances aie ex- 
pressed by the extent of their power to 
raise the temperature of a known quantity 
of water. 

A Thermal unit is the standard of compar- 
ison for quantities of heat, and is the amount 
of heat necessary to raise one pound of 
water i° C. 

Specific heat is the quantity of heat it 
takes to raise different substances to the 
same temperature, and is determined in 
three ways : 

a. By melting ice. 

b. By mixtures. 

c. By cooling. 

The melting ice method is based upon the 
fact that it takes to melt one pound of ice 
80 thermal units. This gives a standard ( for 
comparison, as some substances will take 



33 
more thermal units than others to melt the 
ice. 

By mixtures. A known weight of the sub- 
stance whose specific heat is required is 
heated to a certain temperature and then 
immersed in water whose quantity and tem- 
perature are known. From the tempera- 
ture of the water after mixing, the specific 
heat is determined. 

The cooli?ig method is based upon the fact 
that bodies of different specific heat will oc- 
cupy different lengths of time in cooling 
through the same number of degrees. 
Those that have the greatest specific heat 
will take the longest to cool. Water and 
fluids in general have greater specific heat 
than metals. 

The specific heat of water is taken as 
unity. 

The specific heat of substances increases 
with the temperature, and especially as they 
near their fusing point or change of state. 



CHAPTER VIII. 

SOURCES OF HEAT AND COLD. 

Sources of heat : 

i. Mechanical. 

Compression. 

Friction. 

Percussion. 

Heat due to percussion is not the result 
of compression, but to a vibratory motion 
given to the molecules. 

2. Physical. 

a. Solar radiation. 

b. Terrestrial heat. 

c. Electricity. 

d. Change of state. 

At a certain depth below the surface of 
the earth there is a constant temperature 
below this the temperature increases on the 
average of i° C. for every ninety feet o* 
descent. 

3. Chemical. 

a. Combustion. 

b, Animal heat, 



35 

c. Vegetable heat. 

The heat of combustion depends upon the 
quantity, while the temperature upon the 
rapidity, of the combustion. 

Vegetable heat is more marked in plants 
at the time of blossoming when oxidation 
accompanies the process. 

The process of vegetation in general is not 
accompanied by heat for it is not an oxida- 
tion. 

Under the influence of the sun's rays the 
green parts of the plant decomposes the 
carbonic acid in the atmosphere into oxy- 
gen and carbon which unite with the ele- 
ments of water to form cellulose, sugar, 
starch, &c. To effect this, heat is required 
which is stored up in the plant and reap- 
pears during combustion of wood, coal, Sec, 
arising from its decomposition. 

SOURCES OF COLD. 

a. Change of state. 

b. Expansion of gases. 

c. Radiatien. 

Especially nocturnal radiation, 



CHAPTER IX. 

MECHANICAL EQUIVALENT OF HEAT. 

Heat is the result of motion, and it is 
found, in all cases in which motion is the 
direct producer of heat, that mechanical 
force is consumed. 

By numerous experiments it has been 
found that the mechanical equivalent of heat 
is 772 foot pounds Fahrenheit scale or 1390 
foot pounds Centigrade scale. 

In other words heat and mechanical 
energy are convertible and heat requires for 
its production, and produces by its disap- 
pearance, mechanical energy in the ratio of 
772 foot pounds for every thermal unit. 

WORK OF ENGINES. 

Multiply together the mean pressure on 
the piston, the area of the piston and the 
length of stroke the result will equal the 
working power of the engine. 

This work is compared to horse power 
which represents 550 foot pounds raised one 
foot per second. 

A foot pound is the work necessary to 
raise one pound a foot high, 



BOOK III. 

CHAPTER I. 

LIGHT. 

Theories. 

i. Emission theory. This theory assumes 
that all luminous bodies emit an imponder- 
able substance which penetrating the eye 
acts upon the retina causing - vision. 

2. Undnlatory theory. This theory as- 
sumes that all space and substances are 
filled with a luminiferous ether and the lu- 
minosity of any body is due to the rapid 
vibratory movement of its molecules, thus 
setting in motion this ether which in turn 
acting upon the retina of the eye causes 
vision. 

The undulatory theory is the one usually 
adopted. 

PROPAGATION OF LIGHT. 

Bodies may be : — 

Luminous. 

Transparent. 



38 

Translucent. 
Opaque. 

a. Luminous bodies are those that admit 
light. 

b. Transparent bodies, those that trans- 
mit light so objects can be distinguished 
through them. 

c. Translucent bodies, those that transmit 
light imperfectly so that objects are not 
distinguishable through them. 

d. Opaque bodies are those that do not 
transmit light. 

A luminous ray is the direction of the line 
in which light is propagated. 

A pencil of light is a collection of rays. 
Rays of light may be : — 

a. Parallel. 

b. Divergent. 

c. Convergent. 

& homogeneous medium is one whose chem- 
ical composition and density is the same in 
all parts. In homogeneous mediums light is 
propagated in straight lines. 

SHADOWS. 

Shadcnv is the space behind an opaque 
body upon which light is thrown. 



39 
Penumbra is the dark space around the 
true shadow of an object, and is due to the 
light coming from a source greater than a 
single point. 

VELOCITY of LIGHT. 

The velocity of light has been obtained by 
means of astronomical observations in no- 
ting the difference in time of occultation of 
one of Jupiter's moons when the earth is on 
opposite sides of its orbit. 

This difference has been found to be 
i6'— 36", which represents the time it takes 
light to travel across the earth's orbit and 
gives for its velocity nearly 190.000 miles a 
second. 



CHAPTER II. 

INTENSITY OF LIGHT. 

The following law governs the intensity 
of light. 

The intensity of light is inversely as the 
square of its distance from the source of light 

Photometers are instruments for measur- 
ing the relative intensities of light. 

The sperm candle burning two grains per 
minute is taken as the standard lor this 
measurement. 

Most photometers as Rumford's and 
Bunsens ate made upon the principle that 
to obtain equal shadows light must be of the 
same relative intensity. Therefore by plac- 
ing the sources of light nearer or farther 
away from the object casting the shadows 
until they — the shadows are equal ; the in- 
tensity of the light is determined by the 
law that the intensity of light varies in- 
versely as the square of the distance. 

Thus if one light is a foot away and a se- 



4i 
cond light two feet away from the object 
casting equal shadows, the light two feet dis- 
tant has four times the intensity of the one 
only a foot away. 



CHAPTER III. 

REFLECTION OF LIGHT. 

The reflection of light is governed by the 
following law. 

The angle of reflection is equal to the angle 
of incidence both rays being in the same plane 
and perpendicular to the reflecting surface. 

By drawing a perpendicular to the re- 
flecting surface at the point where the rays 
meet it, the angle of incidence is the angle 
these rays make with the perpendicular, 
while the angle of reflection is the angle re- 
flected rays make with the perpendicular. 

The intensity of the reflecting power of a 
body increases with the degree of polish 
and with the obliquity of the incident ray. 

REFLECTING SURFACES. 

Mirrors are bodies with polished surfaces 
which show by reflection, objects present- 
ed to them. The image is the place where 
the objects appear. 



43 

The surfaces of mirrors are : — 

Plane. 

Curved. 

i. Plane mirrors. In plane mirrors the 
image is formed behind the mirror at a dis- 
tance equal to that of the object forming it, 
and on the perpendicular let fall from this 
object to the mirror. The image is erect y 
virtual and eg ua I in size to the object form- 
ing it 

A virtual image is one formed by the 
prolongation of the reflected rays and not 
by these rays themselves. 

A real image is one formed directly by 
the reflected rays, and not by their pro- 
longation. 

Multiple images are formed by proper ar- 
rangement of several mirrors, as for exam- 
ple in the kaleidoscope. 

II. Curved Mirrors. Curved mirrors 
may be : — 

Spheroidal. 

Parabolic. 

A. Spheroidal mirrors. The centre of 
curvature is the centre of the sphere of 
which the mirror forms a part. 



44 

The principal axis is the line drawn from 
the centre of curvature to the centre of the 
mirror. 

The secondary axis is a line drawn from 
the centre of curvature to any point on the 
mirror. 

The focus is the point at which the re- 
flected rays meet. 

The principal focus is the point where re- 
flected, parallel rays meet. 

The conjugate focus the point where 
other than parallel reflected rays meet. 

Spheroidal mirrors are : — 

Concave. 

Convex. 

a. In concave mirrors which tend to con- 
verge all reflected rays, the principal focus 
is situated midway between the centre of 
curvature and the mirror. 

The position of the image depends upon 
the relation of the object to the mirror. 

i. If the object is at such a distance that 
the rays from it are parallel, the image will 
be situated at the principal focus — real, 
smaller than the object, and inverted. 

2. If the object is beyond the centre of 



45 
curvature, the image will be real, inverted, 
smaller than the object itself and situated 
between the centre of curvature, and the 
mirror. 

3. If the object is between the centre of 
curvature and mirror but nearer the centre 
of curvature, the image will be real, inverted, 
larger than the object and beyond the cen- 
tre of curvature. 

4. If the object is between the centre of 
curvature and mirror but near the mirror, 
the image will be erect, virtual and larger 
than the object. 

b. In convex mirrors, all reflected rays 
tend to diverge, whatever be the position 
of the object in relation to the mirror the 
image is always virtual, erect and smaller 
than the object itself. 
B. Parabolic mirrors. 

In parabolic mirrors, when the light is 
placed at the principal focus the reflected 
rays ate nearer parallel than they are in the 
simple concave mirrors. The light thus re- 
flected maintains its intensity for a greater 
distance, for it is the divergence of the rays 
that diminishes the intensity of light. 



4 6 

Parabolic mirrors are used as head-lights 
in engines, for lantern reflectors, &c, on 
account of the reflected rays being parallel 
and hence maintaining their intensity. 

SPHERICAL ABERRATION. 

When concave mirrors are large the rays 
that are reflected from near the edges meet 
the axis of the mirror nearer the mirror 
than those that are reflected from the cen- 
tral portions. As a result the image is not 
precise or there is spherical aberration by 
reflection. 



CHAPTER IV. 

REFRACTION. 

Refraction of light is the bending of the 
ray of light in passing from one medium to 
another. 

In passing from a denser to a rarer 
medium the ray is bent from a perpendicu- 
lar drawn to the surface of the medium at 
the point of emergence, but toward the 
perpendicular in passing from a rarer to a 
denser medium. 

There is also in refracted light zxv angle of 
ijicidence a?id refraction. 

It a perpendicular be drawn to the sur- 
face separating the two media at the point 
where the rays enter it, Wit angle of inci- 
dence is the angle made by the ray and the 
perpendicular ; and the angle of refraction 
is the angle made by the prolongation of 
the perpendicular and the bent ray as it 
passes through the second medium. 

The incident and refracted rays are in the 



same plane and perpendicular to the sur- 
face separating the two media. 

Most substances only refract the inci- 
dent ray once, but some crystalline bodies 
cause double refraction. 

The index of refraction is the ratio be- 
tween the sines of the incident and refract- 
ed angle. When a luminous ray passes 
from a greater to a less refracting medium, 
at a certain obliquity of the ray the angle 
of incidence is so great that the refracted 
ray emerges parallel to the separating sur- 
face, and the critical angle is the angle this 
oblique ray makes with the perpendicular. 
Any greater obliquity of the incident ray 
will cause total reflection and none of the 
rays will pass out of the first medium. 

MIRAGE. 

Mirage is an optical illusion by which in- 
verted images are seen as if below ground 
or in the atmosphere. It is a phenomena 
of refraction and due to the unequal density 
of different layers of air when they expand 
by contact with the heated soil — the least 
dense rays being the lowest. 



CHAPTER V. 

TRANSMISSION OF LIGHT. 

Light may be transmitted through : — 

Media with parallel faces. 

Prisms. 

Lenses. 

I. When light is transmitted through 
media with -parallel faces the emergent rays 
are parallel to the incident rays. 

II. A prism is any transparent medium 
between faces inclined to each other. Any 
ray of light passing through a prism is re- 
fracted twice in the same direction, once in 
passing into the prism and then again on 
emergence. 

The angle of deviation is the angle this 
emergent ray makes with the incident. 

III. Lenses may be considered as made 
up of a number of prisms. Lenses are 
named from their shape : — 

Convex. 

Plano-convex. 

Concave. 



5° 

Plano-concave. 

Diverging concavo-convex. 

Converging concavo-convex. 

The curved surface of lenses are arcs of 
circles. 

The focus of a lens is the place the re- 
fracted rays meet. 

The real focus is that formed on the op- 
posite side of the lens from the luminous 
rays. 

The virtual focus is that formed on the 
same side of the lens as the luminous rays. 

In convex lenses the principal focus or the 
point where parallel rays meet corresponds 
nearly to the centers of curvature of the 
lens. Convex lenses tend to converge nil 
luminous rays passed through them. 

The position of the image depends upon 
the distance away the object is from the 
lens. 

a. If the object is at such a distance that 
the rays proceeding from it are parallel, the 
image will be real, inverted, smaller than the 
object and situated at the centre of curva- 
ture of the lens. 

b. If the object is beyond the centre of 



5i 
curvature the image will be real, inverted, 
smaller than the object and situated a little 
beyond the principal focus. 

c. If the object is just beyond the cen- 
tre of curvature the image will be real, in- 
verted, larger than the object and situated 
some distance beyond the principal focus. 

d. If the object is between the centre of 
curvature and the lens the image will be 
virtual, erect, larger than the object and 
situated beyond the principal focus. 

Concave lenses. All rays passing through 
concave lenses tend to diverge, hence the 
image is always erect, virtual and smaller 
than the object. 

SPHERICAL ABERRATION IN LENSES. 

When the lens is large the rays which 
are transmitted near the edge reach a focus 
nearer the lens than those rays which pass 
through near the axis, consequently the 
image is indistinct or there is spherical aber- 
ration by refraction. 



CHAPTER VI. 

DISPERSION AND ACHROMATION OF LIGHT. 

When white light or that which reaches 
us from the sun passes from one medium 
into another it is decomposed into several 
kinds of light. This decomposition is call- 
ed dispersion of light. 

Thus if a pencil of solar light pass through 
a prism and be thrown upon a screen, seven 
principal colors are pictured there which 
are from above downward, violet, indigo, 
blue, green, yellow, orange and red. These 
colors thus produced are called the solar 
spectrum. The violet is the most refrangible 
color, and the red the least. 

The spectra formed by artificial light sel- 
dom contain all the colors of the solar spec- 
trum, but those present are invariably in 
the same order as those composing that 
spectrum. The colors of the spectrum are 
simple, that is they cannot be further "de- 
composed. If however the various colors 



53 
are recombined the result is white light 
again. 

The color of bodies depends upon the fact 
that one portion of the colored rays con- 
tained in white light is absorbed, while the 
other portion is reflected, giving them their 
color. 

The bodies that reflect all the colors are 
white while those that reflect none are 
black, mixed colors are those produced by a 
combination of two or more colors. 

Complimentary colors are those which when 
combined produce white. Homogeneous 
light is light containing but one color or 
monochromatic light. 

Common salt burnt in Bunsen's lamp 
gives nearly a homogeneous light. 

PROPERTIES OF THE SPECTRUM. 

a. Luminous properties. Light in the 
yellow portion of the spectrum has the 
greatest and in the violet, the least inten- 
sity. 

b. Heating properties. The hottest por- 
tion of the spectrum is just beyond the 
red, but depends somewhat upon the kind 
of prism used to effect the dispersion. 



54 
c. Chemical properties are more marked 
just beyond the blue portion of the spec- 
trum. 

DARK LINES IN THE SPECTRUM. 

The color of the solar spectrum is not 
continuous, but for several grades of re- 
frangibility rays are wanting and in conse- 
quence it contains many dark lines. These 
lines are always in the same position. 

In spectra of artificial lights and stars 
the positions of the dark lines are changed 
or replaced by bright ones, when compared 
with the solar spectrum. 

Fraunhofers lines is the name given to the 
most marked dark lines in the solar spec- 
trum, and are designated by letters of the 
alphabet. 

A very delicate means of detecting some 
of the ingredients in unknown substances, 
is afforded by volatilizing a portion of the 
substance in a flame and denoting the color 
it imparts to certain of Fraunhofer's lines. 

This method of examination is called 
spectrum analysis. It has been ascertained 
that whenever salts of the same metal are 
introduced into a flame they always produce 
lines identical in color and position. 



55 

SPECTROSCOPE. 

The spectroscope is an instrument used for 
the study of the spectrum and is composed 
of three telescopes mounted upon a com- 
mon footing. The spectra are formed by- 
burning the substance to be examined in 
front of two of the telescopes by means of a 
Bunsen burner, which gives but little light 
of its own, and thus does not obscure the 
lines in the spectrum to be produced. 

The spectra thus formed are brought by 
means of a prism so they may both be seen 
by looking through the third telescope and 
a comparison made, thus detecting the sub- 
stance under examination by means of their 
bright lines. For example if sodium is sus- 
pected of being present in a given substance, 
burn a portion of it in a Bunsen flame in 
front of one of the telescopes, and a little 
sodium in front of the other, and if the bright 
lines of the two agree the supposed sub- 
stance is present. Or, by comparing the 
spectrum of the given substance by the solar 
spectrum, and finding a very bright yellow 
line or band in exactly the same position as 
Fraunhofer's line D. and part ofthe colors 
absent, sodium is present for incandescent 



56 

vapors have the power of absorbing rays of 
the same refrangibility as they admit, and 
sodium spectra produce just such a dark line 
as D. when its rays are transmitted through 
incandescent sodium vapor. 

It is by this property of incandescent 
gases that explains the dark lines in the 
solar spectrum and lead us to believe that 
the sun is surrounded by incandescent gases 
which absorb the rays produced by the 
luminous properties of the same incandes- 
cent substance in the sun. The spectro- 
scope is also used in detecting certain fluid 
as blood and bile. 

This is done by placing some of the fluid 
between the source of light and the spec- 
troscope. The spectrum thus produced has 
characteristic dark bands depending on the 
kind of fluid. 

KINDS OF SPECTRA. 

Continuous. 

Discontinuous or band. 

Absorption. 

a. Continuous spectra are those formed 
by ignited fluids and solids. 

Discontinuous or band spectra are those 
produced by ignited gases. 



57 

Absorption spectra are those produced by 
the fixed stars and the sun. 

FLUORESCENCE. 

Fluorescence is that property some sub- 
stances have of appearing colored when 
viewed by reflected light, but are colorless 
when seen by transmitted light. 

This property is due to the retardation of 
the wave movement by the light being re- 
flected and thus brought within compass of 
the eye. 

ACHROMATIC ABERRATION. 

Simple lenses when a certain distance 
from the eye give colored images, due to the 
unequal refrangibility of the different colors 
or achromatic aberration. To correct this, 
Achromatic lenses or lenses preventing 
decomposition of light are used, and made 
by combining lenses of different substances 
such as crown and flint glass. 



CHAPTER VII. 

OPTICAL INSTRUMENTS. 

Among the most important optical instru- 
ments are the : — 

Microscope. 

Telescope. 

Magic Lantern. 

Camera Obscura. 

a. The simple microscope consists of a sin- 
gle convex lens of short focal length the 
object being placed between the lens and its 
princip.il focus. As a result there is pro- 
duced an erect, magnified and virtual image. 
The compound microscope in its simplest 
form consists of two convex lenses. The 
one nearest the object is called the object 
glass or objective and has a short tocus, the 
other is called the eye piece ox power and 
has a longer focus or is less condensing. 

The object has to be placed very near the 
objective, at or near its principal focAis. 
This forms a real, magnified and inverted 



59 
image which in turn is magnified by the 
eye-piece as is the object in the simple 
microscope. 

b. Telescopes. In astronomical telescopes 
there is an object glass and an eye- piece. 
The object is beyond the principal focus of 
the objective, hence a smaller, inverted and 
real image is formed which is magnified by 
the eye-piece as in the compound micro- 
scope. 

The terrestrial telescope differs from the 
astronomical in producing images in their 
proper positions. This is effected by means 
of two convex lenses placed between the 
objective and the eye-piece so that the im- 
age when magnified by the eye-piece will 
appear erect. 

In Galileo s telescope on the principle of 
which opera glasses are constructed, the eye** 
piece consists of a double concave lens 
which gives at once an erect image and in 
this way differs from the astronomical tele- 
scope. The eye-piece is so placed that the 
image formed by the objective would fall 
behind it, if it were not that the rays in 
passing through the eye-piece are refracted 
and diverge, so that the eye which receives 



6o 

them sees the image near by, magnified and 
erect. 

c. Magic lantern. This is an instrument 
for throwing magnified images upon a 
screen. The reflected rays of a lamp are 
made to fall upon a convex lens which con- 
centrates them upon the object to be magni- 
fied. 

A double convex lens is then placed at a 
little more than the principal focal distance 
from the object and consequently a real, 
magnified and inverted image is produced on 
the screen. 

d. Camera obscura, consists of a dark 
chamber with a small aperture to admit 
the light. 

The rays proceed from external objects 
through this opening and form, small, in- 
verted images of these objects on the op- 
posite wall. The images are made much 
brighter and clearer by placing a double 
convex lens in the aperture. Photographs 
are taken by the Camera, 



CHAPTER VIII. 

PHOTOGRAPHY. 

Photography is the art of fixing images of 
the camera obscura on substances sensitive 
to light. 

The first real important step taken in 
photography was by Daguerre in 1839, and 
in honor to him the pictures thus taken are 
called daguerreotypes. A well polished 
copper plate is made sensitive by exposing 
it to bromine and iodine vapors which 
form iodide and bromide of silver. This 
plate is then placed in the camera at the 
point where the image will be formed and 
allowed to remain a few minutes thus ex- 
posed to the rays of light. 

The plate is next exposed to the action 
of mercurial vapor which deposits metallic 
mercury only on the parts affected by the 
light. All other portions of the iodide and 
bromide of silver are removed from the 
plate by a solution of hyposulphite of soda 



62 

which does not affect the mercury. The 
plate is then washed with a solution of the 
chloride of gold which combines with the 
mercury and increases the lustre of the 
picture. 

Photographs are now taken on glass and 
then transferred to paper. The glass plate 
is first coated with collodion impregnated 
with iodide of potash. This plate is then 
immersed in a solution of nitrate of silver 
thus forming on the plate iodide of silver 
which is very sensitive to light. The plate 
is next exposed to the light in the camera 
as in taking the daguerreotypes. 

& developer ox solution of protosulphate 
of iron is then poured over the plate which 
reduces to the metallic state those parts of 
the iodide of silver acted upon by the light. 
The rest of the iodide of silver is washed off 
the sensitive plate with hypophosphite of 
soda. 

The image thus produced is called the 
negative. The positive picture is obtained 
by impregnating paper with the chloride of 
silver placing over it the negative and ex- 
posing it then to the direct action of the 
sun light for some time. That portion of 



63 
the chloride of silver on the paper, not act- 
ed upon by the light is then removed by 
a solution of hyposulphite of soda. The 
lights of the negative are replaced by shades 
in the positive and vice versa thus making 
the pictures to a certain extent more na- 
tural, taking white, white and black, black. 



CHAPTER IX. 

DOUBLE REFRACTION. 

Some crystals, as iceland spar, have the 
property of splitting up the ray of light 
which passes through them, into two parts 
so that when any object is seen through one 
of these crystals it appears double. 

This property is explained by assuming 
that ether in double refracting bodies is 
not equally elastic in all directions so that 
the vibrations at right angles to each other 
are transmitted with unequal velocities. 
One of the rays into which the incident ray 
is divided is called the ordinary, as it re- 
presents the ordinarily refracted ray, and 
the other the extraordinary ray, as it differs 
from commonly transmitted light. 

The images produced by double refraction 
are called the ordinary and extraordinary 
from the rays that produce them. 

The ordinary rays and images, correspond 
to those of single refraction. 



65 

INTERFERENCE OF LIGHT. 

When two luminous rays from different 
sources meet at acute angles the mutual 
action produced is called interference of 
light, and as a result well marked colors and 
bands are formed which depend wholly up- 
on the meeting of these rays. 

This phenomenon produced by the inter- 
ference of light is due to the meeting of 
the waves of luminiferous ether and com- 
ing from different sources either weaken or 
strengthen each other. As the waves of 
luminiferous ether have different lengths 
and velocities for each color this interfer- 
ence of two rays consequently causes a dif- 
ferent display of colors than would have 
been produced by either. 

The red rays have the longest and the 
violet the shortest wave lengths. 

POLARIZATION OF LIGHT. 

Light besides undergoing double refrac- 
tion is sometimes fiolarized, that is, made to 
vibrate in one plane only. 

Thus if a ray of light which has under- 
gone ordinary refraction be allowed to pas's 
through a second crystal it may be divided 



66 

into ordinary and extraordinary rays with 
unequal intensities, so by rotating the se- 
cond crystal one ray will disappear entirely 
and the other increased in brightness. The 
same is true with the ray which has under- 
gone extraordinary refraction if made to 
pass through the second crystal. 

When light is polarized the ordinary and 
extraordinary rays are polarized in planes 
at right angles to each other. 

MEANS OF POLARIZING LIGHT. 

Light may be polarized by — 

a. Reflection, 

b. Single refraction. By means of a ray of 
light passing through a bundle of glass 
plates. 

c. Double refraction. By this means of 
polarizing light two prisms are needed. 
One is called an analyser, for by means of 
rotating it the ordinary and extraordinary 
rays are made to appearand disappear, thus 
indicating that the light is polarized, and 
the other the polarizer by means of which 
the light is polarized. 

The best prisms used for polarization are 
made of tourmaline which has the property 



6 7 

of absorbing the ordinary ray, and iceland- 
spar which also, as in Nicol's prism, has the 
property of transmitting only one beam of 
the polarized light. 

INTERFERENCE OF POLARIZED LIGHT. 

When rays of polarized light meet they 
cause interference as do the rays of ordina- 
rily refracted light and as a result are pro- 
duced fringes of light with various colors, 
these colors depending upon the planes of 
polarization. 

" ROTATORY POWER OF LIQUIDS. 

A great many liquids as saccharine solu- 
tions, possess the power of rotating the 
plane of polarized light either toward the 
right or the left, and by noting the direction 
nd the number of degrees this plane is ro- 
tated from its original position the compo- 
sition of different substances may be de- 
tected when chemical analysis fails. 

SoleiVs Saccharimeter is an apparatus the 
principle of which is based on the rotatory 
power of saccharine solutions. 

By noting the number of degrees any 
ariven saccharine solution causes the plane 
of polarized light to rotate the amount of 
sugar can be approximately obtained. 



CHAPTER X. 

SOURCES OF LIGHT. 

Light is furnished us by : 

a. Heavenly bodies, 

b. Chemical combustion, 

c. Electricity, 

d. Phosphorescence, 
Phosphorescent substances are those that 

after exposure to certain conditions will 
shine in the dark. This phenomenon may 
be referred to the following causes : 

a. Spontaneous phosphorescence, or 
that seen in vegetables and animals. 

b. Elevation of temperature, 

c. Mechanical effect, 

d. Electricity. 

e. Exposure to the sun's rays. 

The sulphate of calcium and strontium 
are two such substances and will shine in 
the dark for many hours after exposure to 
strong light. 



BOOK IV. 

CHAPTER I. 

ELECTRICITY AND MAGNETISM. 
MAGNETISM. 

Magnets are bodies that attract iron 
and the force exerted is called magnetism. 

Theory of magnetism, 

In all bodies two magnetic fluids, called 
positive and negative, are circulating in op- 
posite directions among the molecules. In 
most substances these fluids neutralize 
each other, but in magnets the positive fluid 
collects at one end of each molecule and 
the negative at the other end. 

KINDS OF MAGNETS. 

Magnets are either : 

Natural, 

Artificial, 

a. A natural magnet or loadstone is an 
oxide of iron Fe3 O4. 

b. Artificial magnets may be — 



7o 

i. Temporary, or 

2. Permanent. 

Temporary magnets are made of iron 
which assumes the property of magnets by 
passing an electric current around the iron. 
(see chapter on electro magnets) or by mag- 
netic induction. 

After the exciting cause is removed the 
iron loses its magnetic properties. 

Permanent \ magnets are made of steel 
which assumes the magnetic property by 
passing a strong current of electricity 
around the steel or by rubbing the steel 
with a magnet for some time. 

After the exciting cause is removed the 
steel retains its magnetism. 

PROPERTIES OF MAGNETS. 

Magnets have the following properties : — 
Attracting iron, 
Assuming a given direction, 
Declination, 
Inclination, or dip. 
Magnetic induction, 
Magnetic rupture. 

a. The attracting force of a magnet is 
more marked the nearer the ends are ap- 



71 
proached, and diminishes toward the cen- 
tre at which point it is absent. 

The poles of a magnet are near the ends 
where the attracting force is the greatest. 

The neutral point is at the centre of the 
magnet where the attracting power is noth- 
ing. 

b. A magnet or magnetic needle, when 
freely suspended, assumes a given direction 
with its poles always pointing in the same 
direction. The poles pointing toward the 
north is called the north pole, while the one 
toward the south is called the south pole. 

Poles of the same name repel each other 
while those of opposite name attract each other. 

An astatic needle is one not influenced by 
the earth's magnetism and is usually a com- 
bination of two needles with poles in con- 
trary directions. 

c. Declination of magnetic needle. The 
north and south poles do not point geo- 
graphically north and south. The amount 
of declination however is not constant but 
differs in places and from time to time. 

d. Inclination or dip of the magnetic 
needle is its tendency when freely suspend- 
ed to incline from jthe horizontal plane. 



72 

North of the Equator the north pole dips 
as the geographical pole is approached un- 
til, before that point is reached, the needle 
becomes vertical in direction. 

The same is true of the south pole south 
of the Equator. Those points where the 
needle assumes the perpendicular, are call- 
ed the magnetic poles of the earth. 

e. Magnetic i?iduction is that property 
which magnets have of temporarily making 
magnets of iron when brought directly or 
nearly in contact with the iron. Such a 
temporary magnet has in turn, but to a 
lesser extent, power of induction on other 
iron. 

As soon as the influence of the perma- 
nent magnet is taken away the iron loses its 
magnetic properties. 

f. Rupture of magnets. When a magnet 
is broken into a number of pieces every 
portion will become immediately a com- 
plete magnet in itself. 



CHAPTER II. 

ELECTRICITY. 

Electricity is a physical agent manifesting 
itself by attraction, repulsion, heat, light, 
&c. Not inherent in bodies but evoked by 
a variety of causes as friction, chemical ac- 
tion, &c. 

Theories. 

i. Franklin s theory. All substances 
contain an imponderable fluid in a quiet 
state but when excited as by friction a por- 
tion of this fluid is drawn off, leaving a pre- 
ponderance of the other portion which 
causes the phenomena of electricity. 

2. Timmers theory. All substances con- 
tain two fluids which ordinarily neutralize 
each other, but when excited are separated 
and cause the manifestations of electricity. 

Admitting this fluid theory of electricity 
any body may be charged with an excess of 
either one fluid or the other, which are call- 
ed positive and negative fluids, and from 
which we have the following law : Two 



74 
bodies chargedwith the same electricities repel 
each other but with oppjsite electricities attract 
each other. 

CONDUCTORS AND NON-CONDUCTORS. 

Bodies in which electricity passes rapidly 
from one part to another are called good 
conductors, but those bodies in which it does 
not, are called poor conductors ox non-conduc- 
tors. Glass is the best non-conductor. Poor 
conductors are also called insulators. 



CHAPTER III: 

KINDS OF ELECTRICITY. 

There are two principal kinds of electric 
city: 

i. Statical, 
2. Dynamical. 

STATICAL ELECTRICITY; 

Statical or frictional electricity was the 
first kind of electricity discovered, and is 
produced by friction. 

PROPERTIES OF STATICAL ELECTRICITY. 

Statical electricity has the properties of — 
Surface distribution, 
Rapid dissipation, „ 
Induction. 

a. The distribution of electricity in any 
body charged with statical electricity, is 
upon the surface and has the greatest in- 
tensity at the most acute points on that 
surface. 

b. Statical electricity is rapidly dissipa- 
ted from bodies even when the}'' are proper- 



76 

ly insulated — that is — mounted on non-con- 
ductors. 

Electricity is dissipated by — 

The air, 

Imperfect insulation. 

c. There is itiditction by statical elect) icity 
as there is induction by magnets. Thus 
when an insulated conductor is charged 
with electricity, any body in a neutral state 
and insulated, placed near it will be acted 
upon in a manner analogous to that of a 
magnet upon soft iron. In other words it 
decomposes the neutral fluid in the body 
attracting the opposite and repelling the 
like kind of electricity. This body will in 
turn have a similar action on other insula- 
ted bodies but all will have their electric 
fluids neutralized again by withdrawing the 
influence of the charged conductor. 

The different kinds of electric fluid — 
negative and positive — are collected at the 
extremities of these bodies charged with 
induced electricity, while at the centre is a 
space destitute of free electricity. 



CHAPTER IV. 

ELECTRICAL MACHINES. 

Among the most important of Statical 
electrical machines are the — 

Electroscope. 

Glass plate machines, 

Electrophorus, 

Leyden jar. 

a. The electroscope is an instrument used 
for detecting the presence and kind of 
electricity in a body. 

This consists essentially of an insulated 
metallic rod at the end of which two gold 
leaves are attached. 

If any electrified body is brought near 
the rod, the electric fluid will be decompos- 
ed by induction, and consequently the gold 
leaves will be charged with one kind of 
electricity, causing them to diverge, while 
the opposite end of the rod will be charged 
with the other kind. 

The presence of electricity is thus deter- 
mined^ the divergence of the gold leaves 



78 

but not the kind. This is determined 
follows. While the rod is still charged by- 
induction from an electrified body, touch 
the end of the rod. This will remove one 
kind of electricity and leave the rod charg- 
ed with the opposite or other kind. After 
removing the finger take away the reducing 
agent and~apply to the rod a positively elec- 
trified body, as an excited brass rod. 

If the gold leaves diverge more widely the 
former body was negatively charged, for like 
electricity will be repelled to the opposite 
extremity. The reverse is true if the gold 
leaves"are less divergent. 

b. Plate electrical machines are instru- 
ments used principally for storing up stati- 
cal electricity. The usual form of this ma- 
chine consists of a circular plate of glass at 
the upper and lower parts of which are two 
cushions which act as rubbers when the 
plate is turned. In front of the plate are 
two isolated conductors terminating in 
branches which are bent around the plate 
between the rubbers and studded with 
points projecting toward the glass. Wh,en 
the plate is turned or revolved it becomes 



79 

:harged with positive electricity by friction 
igainst the rubbers. To equalize this flesi- 
'ive electricity on the glass plate, ?iegative 
electricity is given off from the conductors 
:hrough the projecting points, thus leaving 
the conductors charged with positive elec- 
tricity. 

The cushions are connected with the 
earth by a metallic chain so that the nega- 
tive electricity from the plate can escape to 
the ground. Holtz invented a machine by 
which a body is electrified by friction once 
for all, and made to act by induction in 
such a way as to produce a continual gen- 
eration of electricity, by the rotation of a 
glass plate. The electrophorus acts on the 
same principle. 

c. The electrophorus consists of a disc of 
resin and a metallic disc with an insulated 
handle. The resin is electrified by friction 
and the metallic plate then placed upon it 
which in turn becomes charged with elec- 
tricity by induction from the resin, If this 
plate still in position is touched by the fin- 
ger the like kind of electricity passes to the 
ground while the plate remains charged 



So 

with the opposite kind. If now removed 
from the resinous disc by means of the in- 
sulated handle it still contains but one 
kind of electricity and by touching it a 
spark denotes the union or the neutraliz- 
ing of the two fluids. 

This can be repeated a number of times 

without again exciting electricity in the 
resin. 

CONDENSATION OF ELECTRICITY. 

A condenser is an apparatus for storing up 
a large quantity of electricity on a small 
surface. 

There are a great many forms of conden- 
sers, but the Leyden jar is the most con- 
venient. 

d. The Leyden jar consists of a glass 
bottle, the interior of which is coated with 
tin foil, and the exterior to the neck of the 
bottle with the same material. The neck 
of the bottle is provided with a cork 
through which passes a brass rod termina- 
ting externally in a knob and internally by 
a chain communicating with the tin foil. 

The jar is charged by connecting the ex- 
ternal surface with the ground and the in* 



8i 

terior, by means of the brass knob with the 
source of electricity. In this way the inner 
surface of the jar becomes charged with 
one kind of electricity and the outer by the 
opposite kind, owing to the law of attrac- 
tion and repulsion of electrical fluids. 

By connecting the outer with the inner 
coatings of the jar, by means of a good con- 
ductor, the two fluids unite or the jar is dis- 
charged as indicated by a spark. 

It is found by experiment that nearly 
all the charge of electricity resides on the 
inner and outer surfaces of the glass and 
not on the tinfoil, the. mutual attraction of 
the two opposite electricities causing them 
to approach as nearly as possible, their 
union being prevented by the poor con- 
ductivity of the glass. 

The extent of charge for a Leyden jar de- 
pends directly upon the extent of surface 
and inversely upon the thickness of the 
insulator — glass. 



CHAPTER V. 

EFFECTS OF ELECTRICAL DISCHARGE. 

The electric discharge is the phenomena 
which accompanies the union of two oppo- 
site electric fluids and may be either contin- 
uous or sudden, the difference being one of 
degree and not of kind. 

The effects of the discharge may be 
either — 

Physiological, 

Luminous, 

Heating, 

Magnetic, 

Mechanical, or 

Chemical. 

a. The physiological effects consist in a 
violent exciting action on the sensible and 
contracting elements of the organic tissues 
through which it passes. 

b. Luminous effects often accompany the 
recombination of the two electrical fluids 
and manifested by flashes or sparks of light, 



83 

the color depending upon the surrounding 
medium. Its spectrum is full of dark lines. 
:. Heating effects. The electric spark is 
the source of intense heat. Gas can be ig- 
nited, alcohol set on fire, &c, by the spark 
coming in c©nt?ct with it. 

d. Magnetic effects. By passing the dis- 
charge through a coil of wire inside of 
which a steel needle is placed, the needle 
will afterward be found magnetic. 

e. Mechanical effects consist in lacera- 
tions, fractures, &c, which follow a power- 
ful discharge of electricity through a poor 
conductor, 

f. Chemical effects result in decomposi- 
tion and recombinations, produced by the 
passage of the electrical discharge through 
substances. 

Thus gases in proper proportions are 
made to combine &c, by passing a spark 
through them. 



CHAPTER VI. 

DYNAMICAL ELECTRICITY. 

The discovery of Dynamical or current 
electricity is due to Galvani whose inves- 
tigations on the influence of electricity on 
animals especially the frog, led to additional 
discoveries as to the nature of this form of 
electricity. 

Volta attributed the contractions of the 
frog's legs in Galvani's experiments to the 
metal conductor, used to excite these con- 
tractions, so he constructed the Voltaic pile, 
which consists ofa series of discs of copper, 
zinc and wet cloth arranged in uniform 
order, 

This experiment worked so well that it 
led to further investigations which resulted 
— as the general belief that this form of 
electricity was due to chemical action — i 
the construction of chemical batteries for 
the production of electricity. 



CHAPTER VII. 

CHEMICAL BATTERIES. 

In all batteries the metal acted upon is 
called the positive and the other the negative 
plate. 

If these two plates are connected with a 
wire the direction of the current is deter- 
mined by the positive metal— that is, the 
current always passes from the positive to 
the negative in the fluid, and from the nega- 
tive toward the positive out of the fluid. 

By the direction of the current is always 
meant the direction of the positive current, 
or the one above described. The negative 
current always takes the opposite direction, 
but for all practical purposes it is never 
considered. 

If the wire connecting the two plates be 
divided, the electricity collects at the 
divided ends, hence these are called the 
poles or electrodes. 

The positive electrode is always the one 
connected with the negative plate, while the 



86 

negative electrode is connected with the pos- 
itive plate. 

KINDS OF BATTERIES. 

Among the many batteries constructed 
the following are the most important : 

The simple battery, 

Daniell's battery, 

Grove's battery, 

Bunsen's battery, 

Gravity battery, 

a. The simple battery consists of a plate 
of copper and one of zinc suspended in a 
weak solution of sulphuric acid. The 
chemical action which results causing the 
production of electricity is as follows : — 

Sulphuric acid. Zinc. Hydrogen. Zinc Sulphate* 

H 2 (S0 4 ) + Zn = H 2 + Zn (S0 4 .) 

The zinc is the positive and the copper the 
negative plate. 

This form of battery is seldom used, for 
the current rapidly diminishes until after a 
few hours it ceases to act at all. 

This diminution is due to the following 
causes. 

1. Decrease of chemical action. 



*7 

2. Local action. 

3. Polarization of negative plate. Local 
action is the production of small closed 
circuits of electricity in the positive plate, 
due to impurities. 

This can be prevented to a certain ex- 
tent by amalgamating the zinc, that is rub- 
bing it over with metallic mercury. 

Polarization is the formation of hydrogen 
gas on the copper plate thus interfering 
with the contact between the fluid and the 
metal. The hydrogen also decomposes the 
zinc-sulphate causing a deposit of metallic 
zinc on the copper. 

b. DanielVs battery. This consists of a 
glass vessel filled with a solution of sul- 
phate of copper in which is immersed a 
cylinder of copper and crystals of the sul- 
phate of copper. 

Inside the cylinder is a porous earthen 
cup containing a weak solution of sul- 
phuric acid suspended in which is a cylinder 
of zinc. The chemical action resulting 
is as follows : 

Sulpkuric acid. Zinc. Zinc- Sulphate. Hydrogen. 

H 2 (S0 4 ) + Zn = Zn (S0 4 ) + H 2 



Hydrogen. Copper -sulphate . Copper. Sulphuric acid. 

H 2 + Cu (S0 4 ) = Cu + H 2 (S0 4 .) 
In this way the hydrogen decomposes the 
sulphate of copper, thus preventing; the 
polarizing of the copper plate, and the sul- 
phuric acid formed replenishes that which 
is decomposed by its action on the zinc. 
The sulphate of copper which is decom- 
posed by the action of the hydrogen is re- 
plenished by the undissolved crystals. 

c. Grove s battery. This consists of a glass 
vessel filled with diluted sulphuric acid, im- 
mersed in which is a cylinder of zinc. 

Within this cylinder is a porous earthen 
cup containing nitric acid, suspended in 
which is a plate of platinum. The follow- 
ing chemical action results : 

Zi7ic. Sulphuric-acid. Zinc-sulphate. Hydrogen. 

Zn + H 2 (S0 4 ) = Zn (S0 4 ) + H,. 

Hydrogen. Nitric-acid. Water* Nitrous-acid. 

H 2 + HNO3 — H 2 G + HN0 2 

The nitrous-acid fumes pass off in the 
air. 

d. Bunseris battery is the same as Grove's 
except the platinum is replaced bv a carbon 
plate, the chemical action being the same. 



8 9 

e. The chromic acid batteries are like Bun- 
sen's and Grove's, except the nitric-acid is 
replaced by chromic acid or a solution of 
bi-chromate of potash in sulphuric acid. 
The hydrogen decomposes the chromic acid 
forming oxide of chromium, thus doing 
away with the nitrous acid fumes which are 
disagreeable. 

f. The gravity battery consists of a glass 
vessel in the bottom of which is a copper 
plate covered with crystals of sulphate of 
copper. The vessel is then filled with 
water and a zinc cylinder immersed in it. 
The chemical action is the same as seen in 
Daniell's battery. 

The sulphate of zinc which forms, floats 
on the surface of the sulphate of copper, 
owing to its lower specific gravity and the 
battery thus replenishes itself. In all these 
forms of batteries, a single cell is called an 
element, but a combination of cells ^battery. 
In combining the cells, the positive plate 
in one is connected with the negative in an- 
other by means of a wire, and so on from 
one element to another, until all the cells 
are joined. Increase the number of ele- 



9 o 
ments and in this way increase the strength 
of the current. 

If on the contrary all the same kind of 
plates in the elements are joined together, 
we increase the surface acted upon, and in 
this way the quantity of electricity pro- 
duced is proportionally increased. 



CHAPTER VIII. 

DIRECTION AND MEASUREMENT OF CURRENT 
ELECTRICITY. 

The direction and measurement of voltaic 
currents of electricity are determined by 
the magnetic effects they produce. 

If a wire is suspended horizontally in the 
direction of a magnet, and a current be pass- 
ed through this wire, the needle is deflected 
and tends to take a position which is more 
nearly at right angles to the magnetic me- 
ridian in proportion, as the current is 
stronger. 

If the observer imagine himself the wire, 
the current entering his feet and passing 
out of his head, and his face always toward 
the needle ; the north pole of the needle will 
always be deflected toward the left of the 
current. 

The Galvanometer is an apparatus bv 
means of which the direction and in- 
tensity of currents can be determined and 
is based upon the above principle of deflec- 



9 2 

tion of the needle by the current. If, how- 
ever, instead of having a single wire, a com- 
plete circuit is formed around the needle or 
several such circuits are made, the effect is 
multiplied. 

INTENSITY AND RESISTANCE OF THE ELEC- 
TRO-MOTOR FORCE. 

The Electro-motor force, is that -force by 
which electricity is set in motion, and its 
intensity and resistance are governed by 
the following laws : 

a. The intensity of a current is equal to 
the electro-motor force, divided by the resist- 
ance ; and is inversely proportional to the 
length, and directly to the section of the con- 
ductor. 

b. The resistance of a current is inversely 
proportional to the intensity of the current, 
and section of the conductor and directly as 
the length of the conductor. 

In the ordinary element there are two re- 
sistances to be considered : 

a. Internal ox resistance of the fluid. 

b. External ox resistance due to the pas- 
sage of the current outside the liquid. 



CHAPTER IX. 

EFFECTS OF CURRENT ELECTRICITY, 

The effects of current electricity may be : 

Magnetic. 

Heating. 

Luminous. 

Physiological. 

Chemical. 

a. The magnetic effects have already 
been considered. 

b. Heating effects. The heat disengaged 
in any given time is directly proportional 
to the square of the strength of the cur- 
rent and to the resistance. 

A battery of thirty Bunsen elements will 
melt copper wire, hence the thermal effect 
of the current is used for firing mines, cau- 
terizing, &c. 

The heating effects depend more upon the 
size of the plates than upon the number of 
them when the resistance is not great. 



94 

c. Luminous effects. In making and break- 
ing a voltaic current at the point of contact 
a spark is obtained. If at the two teimi- 
nals of a strong battery, pencils of carbon 
are placed near together, and an intense 
current passed, the pencils become incan- 
descent, and a luminous arc extends be- 
tween the two points called the voltaic arc. 

The electric light has the same chemical 
properties as solar light. Its spectrum has 
several bright lines. 

d. Physiological effects. The voltaic cur- 
rent has the property of causing contrac- 
tion of all protoplasmatic matter, and excite- 
ment throughout the nervous system. 
These effects are only produced at the open- 
ing and closing of the circuit. 

e. Chemical effects. Many substances are 
decomposed into their elements by the vol- 
taic current. This decomposition is called 
Electrolysis. The positive electrodes effect- 
ing this electrolysis is called the anode, and 
the negative, the kathode. 

Many substances previously considered 
elements, have by electrolysis proven to be 
of a compound nature. In the decomposi- 
tion of salts the acids are liberated at the 



95 

anode, and the bases at the kathode. Elec- 
trolysis cannot take place in substances un- 
less they are conductors. 

ELECTRO METALLURGY. 

Electro metallurgy or galvano-plastics, is 
the decomposition of salts by which certain 
metals are precipitated on the negative 
plate. In this way electro gilding is done. 



CHAPTER X. 

ELECTRO DYNAMICS. 

Electro dynamics treat of the laws of elec- 
tricity in a state of motion or the action of 
electric currents on each other and on 
magnets. Two currents parallel and in the 
same direction attract each other, but in con- 
trary directions repel each other. 

Currents impart the same rotatory mo- 
tion to magnets that they do to currents, 
but not only do currents act upon magnets 
but magnets also act upon currents, caus- 
ing them to rotate when the magnet is 
fixed. 

The earth which exercises a directive ac- 
tion on magnets acts also on currents. 
Thus every vertical current tends to place 
itself, under the earth's influence, in a plane 
perpendicular to the earth's magnetic meri- 
dian, but every horizontal current tends to 
have a continuous rotatory movement 
rather than a directive one. 



97 

Not only has the earth a directive ten- 
dency on a freely movable current, but also 
causes, as in the magnet, an inclination or 
dip of a circuit when it is properly balanced. 

MAGNETIZATION BY CURRENTS. 

Electro magnets are bars of soft iron which 
under the influence of a voltaic current be- 
come magnets. But this magnetism is only 
temporary, for so soon as the current 
ceases the iron is no longer a magnet, or if 
at all, a very weak one, the magnetism re- 
maining being called residual. These elec- 
tro magnets are usually made in the horse- 
shoe form, and the insulated copper wire is 
wound around the two armatures in the 
same direction, so as to form bobbins. 

As a general rule electro magnetic force or 
strength is directly proportional to the strength 
of the current, and to the number of windings 
of the wire, but independent of the nature and 
thickness of the wire. 

It is on the principle of the electro mag- 
net that all telegraphing is done. 



CHAPTER XI. 

VOLTAIC INDUCTION. 

Induction by the Voltaic current is the ac- 
tion which currents or magnets exert on 
conductors. 

Induction can be produced by : 

Magnets. 

Interrupted currents. 

Continuous currents. 

A current upon itself. 

a. If a magnet be introduced into the 
centre of a hollow coil of insulated wire, at 
the moment of introduction and withdrawal 
a momentary current will be excited in the 
coil of wire. These excited currents are 
opposite in direction. 

b. Induction by interrupted currents can 
be produced by introducing an insulated 
coil of wire, called primary, into another 
insulated coil, called the secondary, and pass- 
ing a current of electricity through the 
primary. 



99 
At the moment of making and breaking 
this current a secondary curre?it will be ex- 
cited in the secondary coil but in opposite 
directions, viz.: at the moment of making 
in the inverse direction to that in the pri- 
mary coil and in the same direction at the 
time of breaking. 

c. By continuous currents. When a pri- 
mary coil is traversed by a continuous cur- 
rent and brought near a secondary coil or is 
removed from it, currents are excited in this 
secondary coil as seen in (b). 

d. Induction can be excited by a current 
on itself when two portions of the same 
insulated wire are placed side by side, the 
sudden making and breaking of a current in 
one wire will excite a current in the other 
called extra. The making of this current is 
a hindrance to the passage of electricity 
throughout the length of the wire, for the 
extra current is an inverse one, but the 
breaking intensifies the current as it is in 
the same direction as the current itself. 

By placing a core of soft iron in the in- 
terior of any of the coils above mentioned 
for the production of induced currents, the 
effect will be intensified, the iron acting as 



a magnet when the current is transmitted 
around it. 

The following law governs induced cur- 
rents. The strength of the induced currents 
is proportional to the inducing currents and to 
the products of the lengths of the inducing and 
induced currents. 

APPARATUS FOUNDED ON INDUCTION. 

The following apparatus is founded on 
induction : 

Magneto-electrical machines. 

Induction coils. 

Telephone. 

a. The magneto-electrical machine in its 
simplest form consists of a horse shoe mag- 
net around which two bobbins are made to 
revolve. The wire forming these bobbins 
is coiled on two cylinders of soft iron 
jointed by plates of iron or brass, thus 
forming electro-magnets. So that the in- 
duced currents produced, may be in the 
same direction, the wire on the bobbins is 
coiled in opposite directions. By each com- 
plete revolution of the electro-magnet, its 
two armatures become alternately magneti- 
zed in contrary directions, and in each wiie 
an induced current is produced, the direction 



of which changes at each half turn. By 
aid of a communicator the currents may all 
be made to pass in the same direction. 

b. Induction coils, of which Ruhmkorff 's 
is a good example, are arrangements for 
producing induced currents of electricity by 
means of electric currents whose circuits 
are opened and closed alternately in rapid 
succession. 

These coils consist essentially of hollow 
cylinders in which are placed bundles of iron 
wire. Each cylinder has two helices, coil- 
ed around it one connected with the poles 
of a battery, the current of which is alter- 
nately opened and closed by a self-acting 
arrangement, and the other serving for the 
development of the induced current. 

By means of an apparatus of this kind, 
currents of electricity are produced supe- 
rior in effects to those obtained by electrical 
machines or Leyden batteries. In Ruhm- 
korff's machine there is placed in the bottom 
a condenser made of tin foil so insulated that 
each alternate layer, forming one coating, is 
connected with the inducing current, thus 
storing up some of the electricity from the 
battery and extra currents ; while the remain- 



ing layers, forming a second coating, are 
connected with the secondary or induced 
currents, thus increasing their strength by 
using this stored-up electricity. * 

c. The telephone is an instrument whose 
action depends upon the fact, that when- 
ever the relative positions of a magnet and 
a closed coil of wire are altered there is 
produced within the coil a current of elec- 
tricity. The magnet and the coil in the 
telephone remains fixed, but the iron mem- 
brane or diaphragm in the mouthpiece, 
which vibrates backward and forward when 
spoken into, becomes magnetized by induc- 
tion from the magnet. These vibrations 
give rise to currents in the coil surround- 
ing the permanent magnet, which are trans- 
mitted through the circuit to the distant 
coil surrounding its magnet which in turn 
alternately attracts and ceases to attract the 
corresponding diaphragm, thus producing 
similar sounds by exciting vibrations in the 
air. 



CHAPTER XII. 

THERMO-ELECTRIC CURRENTS. 

Currents of electricity can be produced 
by applying heat or cold to one of the 
junctions of a circuit composed of different 
metals. 

The electricity thus produced is called 
Thermo-Electric curre7its . 

In the thermo-electric pile a large number 
of elements or metals are used in a small 
space and alternately soldered together. 

The elements may be of many kinds, or 
better still, many elements of the same 
kind, 



APPENDIX. 

METEOROLOGY. 

Meteorology treats of the phenomena pro- 
duced in the atmosphere. 

By the climate of a place is understood 
the whole of the meteorological condition 
to which the place is subjected. The mean 
temperature for a day is that obtained by ad- 
ding together twenty-four hourly observa- 
tions, and dividing by twenty-four. 

Causes modifying the temperature of any 
place. 

The temperature of any place is modified 
by its : 

Latitude. 

Height. 

Direction of wind. 

Nearness to the sea. 

Nearness to the gulf stream. 

a. The influence of latitude on the tem- 
perature arises from the greater obliquity 
of the solar rays the further North or South 



105 

)f the Equator any place is situated, and 
:onsequentIy a less quantity of heat is ab- 
sorbed. 

The length of day influences the tempera- 
;ure in Northern countries, and makes this 
emperature more uneven than in those 
:ountries where the days and nights are 
nearly equal. 

b. Height. In the temperate zone a di- 
ninution of i°C corresponds in the mean 
;emperature to an ascent of every 180 yards. 

c. Winds coming from hot or cold coun- 
ties influence the temperature of a place 
greatly. 

d. Nearness to the sea tends to raise the 
emperature of neighboring places and ren- 
ler it uniform. 

e. The Gulf stream exerts an influence 
ike the winds from hot countries. 

The temperature of the air on the surface 
)f the Globe is subject to perturbing causes, 
;o purely local and varied, that the isother- 
nal lines encircling the earth are greatly 
:urved. 



INDEX. 




A.. 




Aberration, spherical of mirrors, 


46 


" lenses, 


51 


" achromatic, 


57 


Absorbing hygrometers, 


28 


Absorption spectra, 


56 


Accurate thermometers, 


18 


Achromatic aberration, 


57 


lenses, 


57 


Adhesion, 


7 


Air barometers, 


12 


" pumps, 


13 


" " Sprengel's, 


13 


" thermometers, 


17 


Alcohol 


17 


Analyser, 


66 


Analysis, spectrum, 


54 


Aneroid's barometer, 


12 


Angle, critical, 


48 


of deviation, 


49 


" " incidence in reflection, 


42 


" refraction, 


47 


Angle of reflection, 


42 


" " refraction, 


47 


Anode, 


94 


Apparatus, induction, 


100 



io8 

Arc light, voltaic, 
Archimedes, law of, 
Artificial magnets, 
Astatic needle, 
Astronomical telescope, 
Athermanic bodies, 
Atoms, 

" force controlling, 
Axis, principal, 
" secondary, 



Barometers, 

air, 
" mercury, 

Aneroid's, 
corrections for, 
Batteries, chemical electric, 
kind of, 
local action in, 
polarization in, 
Boiling, 

" point, 
Boyle's laws of gases, 
Bunsen's battery, 

o. 

Calorimetry, 
Camera obscura, 
Centigrade thermometer, 
Change of state. 
Chemical hygrometers, 

effects of current electricity, 
'* " " electric discharge, 



109 



Chemical electric batteries, 


85 


sources of heat, 


34 


Chromic acid battery, 


89 


Coefficient of expansion, 


19 


Cohesion, 


7 


Cold, sources of, 


35 


Color of bodies, 


53 


Colors, complimentary, 


53 


simple, 


52 


mixed, 


53 


Compensation pendulum, 


19 


Complimentary colors, 


53 


Compound microscope, 


58 


Concave lenses, 


5i 


" mirrors, 


44 


Condensing hygrometers, 


28 


Condensation of electricity, 


80 


Conducted heat, 


3° 


Conductors, 


74 


Continuous spectra, 


56 


Convected heat, 


3 1 


Conversion of thermo-metric degrees, 


16 


Convex mirrors, 


45 


" lenses, 


5° 


Corrections for barometers, 


n 


Critical angle, 


48 


Current electricity, 


84 


effects of, 


93 


" direction of, 


9i 


measurement of, 


9i 


Currents, thermo-electric, 


103 


Curved mirrors, 


43 



ID. 

Daguerre, 

Daguerreotypes, 

Daniell's battery, 

Dark lines in spectrum, 

Declination of magnetic needle, 

Degarager, Otto, 

Delicate thermometers, 

Deviation, angle of, 

Developer, 

Dew point, 

Diathermanic bodies, 

Digestor, Papin's, 

Direction of electric currents, 

Discontinuous spectra, 

Dispersion of light, 

Dissipation of statical electricity, 

Distillation, 

Distribution of statical electricity, 

Double refraction, 

Dynamical electricity, 

Dynamics, electro, 

E. 

Elasticity, 

Electro dynamics, 

Electricity, Franklin's theory of, 

Timmer's 

theories of, 

kinds of, 

statical, 

frictional, 

dynamical, 



Electricity, current, 


84 


attractions and repu! 


Isions of, 73 


Emission theory of heat, 


14 


" light, 


37 


Engines, work of, 


36 


Evaporation, 


24 


Expansions of solids, 


J 9 


" liquids, 


19 


" gases, 


20 


Expansion, coefficient of, 


19 


Extraordinary images, 


64 


" ray, 


64 


refraction, 


66 


Eye-piece 


58 


F\ 




Fahrenheit thermometers, 


16 


Fluorescence, 


57 


Foci of lenses, 


5o 


" " mirrors, 


44 


Focus conjugate, 


44 


" principal, 


44 


real, 


5o 


" virtual, 


5° 


Force, electro-motor, 


92 


Foot pound, 


36 


Form, 


5 


Franklin theory of electricity, 


73 


Fraunhofer's lines, 


54 


Frictional electricity, 


75 


Fusion, 

Galileo's telescope, 


21 


59 


Galvani's experiment, 


84 



Galvanometer, 
Galvano-plastics, 
Gaseous state, 
Gases, 

law of, 

specific gravity of, 
" expansion of, 
Gravitation, 

laws of, 
Gravity, 

battery, 
Grove's battery, 

H. 

Heat, 

" latent, 

" conducted, 

" convected, 

" vegetable, 

'• specific, 

" radiated. 

" theories of, 

" transmission of, 

" quantities of, 

" reflection of, 

mechanical equivalent of, 

" sources of, 
Heating effects of electric discharge, 
" current electricity 
Holtz machine, 
Homogeneous light. 

medium, 
Horse power. 



Hydrometers, 9 

Hygrometry, 27 

Hygrometers, 27 

condensing, 28 

chemical, 27 

absorbing, 28 

" wet-bulb (Psychrometers) 28 

I. 

Image, real, 43 

virtual, 43 

multiple, 43 

ordinary, 64 

extraordinary, 64 

Impenetrability, 5 

Inclination of magnetic needle, 71 

Incident angle of reflection, 42 

" refraction, 47 

Index of refraction, 48 

Induction, magnetic, 72 

voltaic, Q 8 

statical electrical, 76 

apparatus, IO o 

coils, IOI 

Instruments, optical, $g 

Intensity of light, 40 

" laws of, 40 

Interference of light, 6> 

of polarized light, 67 

Insulators, 74 



U4 

Kl. 

Kathode, 

Kinds oT batteries, 

" electricity. 

" spectra, 

L. 

Latent heat, 

" of water, 
" vapors, 
Law, Boyle's 
" Pascal's 
" Archimede's, 
" of gases, 

" gravitation, 
" intensity of light, 
" reflection " 
" magnetic attractions, 
repulsions, 
" electrical, attractions, 
" " M repulsions, 

" electro-motor force, 
Lenses, 

foci of, 
" convex, 
*' achromatic, 
•• concave, 

principal foci of, 
Leyden jar, 
Light, 

" iJh theories of, 

*■ transmission of. 

" ray of. 



H5 

Light, pencil of, 38 

" velocity of, 39 

" intensity of, 40 

" reflection of, 42 

" refraction of, 47 

" dispersion of, 52 

" interference of, 65 

" polarization of, 65 

" sources of, 68 

" homogeneous, 53 

Liquids, 9 

Liquid state, 6 

Liquids, specific gravity of, 9 

expansion of, 19 

" rotatory power of, 67 

Liquefaction of vapors, 25 

Local temperature, causes modifying 104 

Local action in batteries, 87 

Luminous effects of current electricity 94 

" electric discharge, 82 

Machines, statical electrical, 77 

" magneto electrical, 100 

Magic lantern, 60 

Magnetism, 69 

theory of 69 

residual, 97 

Magnets, 69 

natural, 69 

artificial, 69 

temporary, 70 

" permanent, 70 



n6 

Magnets, properties of 
" inclination of 
declination of 
neutral point of 
Magnetic poles, 

" induction, 

" rupture, 

attractions and repulsions, 
effects of electric discharge, 
" current electricity, 
Magneto electrical machines, 
Masses, 

forces governing. 
Matter, 

" properties of 
" divisions of 
" forces governing, 
Mean temperature, 
Means of polarizing light, 
Measurement of current electricity, 
Mechanical sources of heat, 

equivalent of heat, 
effect of electric discharge, 
Medium, homogeneous, 
Mercury barometers, 

thermometers, 
Meteorology, 
Microscope, simple 

" compound, 

Mirage, 
Mirrors, 

plane, 
curved, 



U7 

Mirrors, concave, 44 

" convex, 45 

parabolic, 45 

spheroidal, 44 

Mixed colors, 53 

Molecules, 6 

forces governing, 7 

Multiple images, 43 

:sr. 

Natural magnets, 69 

Negative poles of battery, 85 

plates of battery, 85 

pictures, 62 

Neutral point of magnets, 71 

Nicol's prisms, 67 

Non-conductors, 74 

o. 

Object glass, 58 

Objective, 58 

Opaque bodies, 38 

Opera glass, 59 

Optical instruments, 58 

Ordinary ray, 64 

image, 64 

refraction, 64 

Papin's digestor, 24 

Parabolic mirrors, 45 

Pascal's experiment, . n 

Pascal's law 9 

Pencil of light, 38 

Pendulum, compensating, 19 



n8 

Penumbra, 
Permanent magnets, 
Phosphorescence, 
Photography, 
Photometers, 
Physics, definition of 
Physical sources of heat, 
Physiological effects of electric dis- 
charge, 
Physiological effects of current elec- 
tricity, 
Pictures, negative, 
positive, 
Pile, voltaic, 

" thermo-electric, 
Plane mirrors, 
Plate electrical machines, 
Polarity, 
Polarization of light, 

" " " means of. 

" " negative plate, 

Polarized light, interference of, 
Polarizer, 
Poles of magnet, 
" battery, 
Positive poles of battery, 
" metal of battery, 
pictures, 
Power, 

Principal axis of mirror, 
focus of mirror, 
" " '■ lenses, 

Prisms, 



ii 9 



Prism, Nicol's 


67 


Properties of magnets, 


70 


" •"' statical electricity, 


75 


" " spectrum, 


53 


Pumps, air, 


13 


Sprengel's air, 


13 


Psychrometers, 


28 


Pyrometers 


18 


Q. 




Quantities of heat, 


32 


ie. 




Radiated heat, 


3° 


Ray of light, 


33 


" ordinary, 


64 


" extraordinary, 


64 


Real images, 


43 


" foci of lenses, 


5o 


Reaumur's thermometer, 


16 


Reflected heat, 


3 1 


Reflection of light, 


42 


laws of 


42 


(i angle of 


42 


Reflecting surfaces, 


42 


Refraction of light, 


47 


angle of 


47 


index of 


48 


total 


48 


double 


64 


" ordinary 


64 


extraordinary, 


64 


Repulsion, 


7 


Residual magnetism, 


97 



Rotatory power of liquids, 
Rupture of magnets, 
RuhmkorfT's coil, 

s. 

Sacchanmeter Soleil's, 
Saturated vapors, 
Secondary axis, 
Shadows, 

Simple microscope, 
colors, 

electric battery, 
Solar spectrum, 
Soleil's saccharimeter, 
Solid state, 
Solids, 

specific, gravity of 

expansion of 

Sources of heat, 

" cold, 

" light, 

Specific gravity, 

" standards tor 

. " " means of obtaining 

" " of liquids, 

bottle, 
" " of solids, 

" " of gases, 

Specific heat, 
Spectrum, properties of 
solar 
" dark lines in 

analysis, 



Spectroscope, 


55 


Spectra, kinds of 


56 


continuous 


56 


discontinuous 


56 


" absorption 


57 


Spheroidal condition, 


25 


mirrors, 


44 


Spherical aberration of lenses, 


5' 


" mirrors, 


46 


Sprengel's air pump, 


13 


State spheriodal, 


25 


hygrometric, 


27 


Statical electricity, 


75 


properties of 


75 


dissipation of 


75 


induction by 


76 


" distribution of 


75 


electrical machines, 


77 


Surfaces reflecting, 


42 


T. 




Telephone, 


102 


Telescopes astronomical, 


59 


terrestrial, 


59 


Galileo's, 


59 


Temperature, 


14 


Temporary magnets, 


70 


Tension of vapors, 


34 


Terrestrial telescope, 


59 


Theories of heat, 


H 


•Might, 


37 


" electricity, 


73 


" magnetism. 


69 



Thermal unit, 

Thermometers, 

mercury, 

Fahrenheit's, 

Centigrade, 

Reaumur, 

alcohol, 

air, 

delicate, 

accurate, 

Thermometric convertions. 

Thermo-electric currents. 

Thermo-electric pile, 

Torricelli's experiment, 
vacuum, 

Total reflection, 

Translucent bodies, 

Transparent bodies, 

Transmission of light, 

XJ. 

Undulatory theory of heat, 
" light. 
Unit, thermal 

V. 

Vacuum Torricellian, 

Vaporization, 

Vapors, 

saturated 
tension of 
latent heat of 
liquefaction of 



'4 
37 
32 

10 
23 
23 

23 
24 
24 

^5 



123 

Vegetable heat. 35 

Vegetation, process of 35 

Velocity of light, 39 

Virtual image, 43 

foci of lenses, 50 

Voltaic arc, 94 

pile, 84 

induction, 98 

by magnets, 98 

by interrupted currents, 98 

" " continuous " 99 

" a current upon itself, 99 

Water, latent heat of 21 

Wet -bulb hygrometers, (Psychrometers,) 28 

Work of engines, 36 



f 



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